1. Field of the Invention
The present invention relates to a focal-plane shutter for use in photographic cameras and particularly to such a shutter including light-shielding blade means in the form of a flat plate which is adapted to move across an aperture for exposure such that the aperture will be closed by the surface of the blade means.
2. Description of the Prior Art
There is generally known a focal-plane shutter comprising a plurality of shutter blades which are driven by any suitable driving means to move within a space formed by two parallel base plates along the surfaces of these base plates with different movements of the blades so that an aperture will sequentially be closed by the shutter blades. Each of the base plates has an opening formed therein which is normally of a rectangular shape and defines the aperture for exposure. The shutter blades are normally retracted within the space outwardly of one side of the rectangular opening and then moved toward the opposite side of the opening to shield the light. One of the shutter blades which has the largest displacement is rapidly run across the rectangular opening into the portion of the space outside the opposite side of the aperture during the movement for exposure.
To assure the light-shielding property, it is desirable that the above space is limited to the minimum dimension necessary for the shutter blades to run across the aperture. It is therefore important that the shutter blades are maintained flat to assure that the shutter blade is smoothly moved into the portion of the space outwardly of the opposite side of the rectangular opening thereacross immediately before the end of the movement. Recently, a new technique has been developed to decrease the mass of shutter blades themselves so that the shutter blades can more rapidly move to close the aperture throughout the entire area thereof substantially at the same time. Such a technique, however, tends to reduce rigidity of the shutter blades.
If a shutter blade of relatively low rigidity is moved at high speed and then abruptly stopped, part of the kinetic energy in the shutter blade acts thereon to deform the blade itself, resulting in a deflection in the crossing direction of the blade surface. The driving means is normally braked in the course of the movement of the shutter blade across the rectangular opening, that is, before the shutter blade begins to move into the space portion outside the opposite side of the opening. At this point of time, therefore, the shutter blade is deformed in the crossing direction of the blade surface and loses its flatness. Thus the shutter blade fails to move smoothly into the space portion outside the opposite side of the opening. In the worst case, the shutter blade impacts the opposite edge of the base plate, causing damage to the shutter blade.